1. Field of the Invention
The present invention relates to a toner for developing electrostatic latent images, for use in electrophotographic apparatus and electrostatic recording apparatus, and to a developer using the toner, and a process cartridge, an image forming method and an image forming apparatus using the developer.
2. Discussion of the Background
In the electrophotographic apparatus and electrostatic recording apparatus, a toner is adhered to an electrostatic latent image formed on a photoreceptor to form a toner image thereon, and the toner image is transferred onto a transfer material and is fixed thereon with heat. Full-color images are typically reproduced by overlapping a black toner image, a yellow toner image, a magenta toner image and a cyan toner image on a transfer material and simultaneously fixing them with heat thereon.
However, users who are used to seeing printings are not typically satisfied with the image quality produced by a full-color copier yet, and a higher definition and a resolution close to the photographs and printings thereof are required. It is known that a toner having a small particle diameter and a narrow distribution thereof is used to produce high-quality images in electrophotography.
A conventional pulverized toner is prepared by kneading a colorant, a charge controlling agent, an offset inhibitor, etc. in a thermoplastic resin upon application of heat, uniformly dispersing them therein to prepare a mixed composition, and pulverizing and classifying the mixed composition. The pulverized toner tends to have a wide particle diameter distribution, and for example, fine particles having a diameter not greater than 5 μm and coarse particles having a diameter not less than 20 μm have to be removed to produce images having good image resolution and tone reproduction, and therefore a yield extremely decreases. In addition, it is difficult to uniformly disperse the colorant, charge controlling agent and the like in the thermoplastic resin by the pulverization method. Nonuniform dispersion thereof adversely affects fluidity, developability and durability of the resultant toner and image quality produced thereby.
Recently, polymerization methods of preparing a toner have been suggested to get rid of the problems of the pulverization method. The polymerization methods of preparing a toner for developing electrostatic latent images, such as a suspension polymerization method, are known.
The polymerization methods can save the conventional pulverizing and kneading processes to save energy, and can shorten preparation time and improve process yield to reduce cost. Further, the polymerization methods can make the particle diameter of a toner smaller and the particle diameter distribution thereof shaper than a pulverized toner to produce higher-quality images.
In order to produce electrophotographic full-color images having quality similar to that of printings, each color toner needs to have high color reproducibility. In order to achieve this without problems, a colorant having good transparency, light resistance and heat resistance is highly dispersed in a toner.
Conventional colorants for a yellow toner have various problems, e.g., dye colorants typically have poor light resistance and image preservation stability although having good transparency.
Although pigments have light resistance higher than that of dyes, they preferably have light resistance as high as copper phthalocyanine pigments.
Further, yellow pigments having good light and heat resistance has opacifying strength too strong to extremely deteriorate transparency thereof, and which is unsuitable for producing full-color images.
Japanese Patent Publication NO. 2-37949 discloses a disazo compound having good light resistance and a method of preparing the disazo compound. This is a compound group typified by PIGMENT YELLOW 180 which is one of azo pigments in compliance with the recent safety requirement.
Japanese Laid-Open Patent Publications Nos. 6-230607, 6-266163 and 8-262799 disclose toners using the PIGMENT YELLOW 180, which do not have good colorability nor good transparency, but the pigment is often used for pulverized toners.
In a chemical toner, a state of existence of a pigment occasionally varies due to its original properties. According to the dispersibility and dispersed stability of the pigment therein, the pigment agglutinates therein or separates out onto the surface thereof when granulated, resulting in variation of fixability and chargeability thereof. The PIGMENT YELLOW 180 having high hydrophilicity is difficult to use in methods of dispersing an oil phase in water such as a suspension polymerization method because of segregating on the surface of a toner or re-agglutinating therein.
Japanese Laid-Open Patent Publications Nos. 2004-212451, 2001-166540, 2001-109196, 11-202558 and 7-230184 disclose PIGMENT YELLOW 155 having a good spectroscopic property, which are sued in various polymerization methods and pulverization methods. Since the PIGMENT YELLOW 155 is not so hydrophilic as the PIGMENT YELLOW 180 and need not be surface-treated, the PIGMENT YELLOW 155 can more easily be used. However, the PIGMENT YELLOW 155 has high agglutinability and poor dispersed stability.
Because of these reasons, a need exists for a toner producing images having sufficient image density as well as having good developability, transferability and fixability without excessive uneven dispersion of a pigment on the surface thereof.